1. Field of the Invention
The present invention relates to a semiconductor laser and a method of manufacturing the same and, more particularly, to a semiconductor laser with good efficiency and a simplified method of manufacturing the same.
2. Related Background Art
Since a semiconductor laser has advantages, e.g., compact size, and easy handling, and the like, it is widely applied to a light source for a laser beam printer, a magnetooptical recording apparatus, an optical communication system, and the like.
In such a semiconductor laser, in order to efficiently emit a laser beam, a so-called stripe structure is inevitable wherein a current is concentrated on a stripe region extending along a resonating direction of a laser resonator. Upon formation of the stripe structure, many lateral p-n junctions are used. As a method of manufacturing the stripe structure, a so-called diffusion method, an ion-implantation method, and the like are known. Of these methods, the diffusion method is performed as follows.
FIGS. 1A, 1B, and 1C are sectional views showing processes in the diffusion method. As shown in FIG. 1A, a 1-.mu.m thick mask 111 of Si.sub.3 N.sub.4 is formed on an n-type GaAs substrate 110. The substrate 110 on which the mask 111 is formed is placed in a ZnAs.sub.2 atmosphere, and is heated at 630.degree. C. for several tens of minutes, so that Zn ions are diffused in the substrate 110 to a depth of about 2 .mu.m, as shown in FIG. 1B, thereby forming p-type regions 112. Thereafter, the mask 111 is removed, and the p-type regions 112 are thus formed in the substrate 110, as shown in FIG. 1C.
The ion-implantation method will be described below.
FIGS. 2A and 2B are schematic sectional views showing processes in the ion-implantation method. As shown in FIG. 2A, a metal mask 114 is formed on an n-type GaAs substrate 113, and the resultant structure is irradiated with a Zn.sup.+ beam 116 to form p-type regions 115. At this time, the crystal of the p-type regions 115 is disordered, and cannot be called a single crystal. Thus, after the mask 114 is removed, a 1-.mu.m thick cap layer 117 of Si.sub.3 N.sub.4 is deposited on the resultant structure, and is annealed at about 800.degree. C. for about 30 minutes, thereby recovering the crystallinity of the p-type regions 115. As ions to be implanted, Si, Se, or S ions, or double implantation of Se and Ga, Ge and Ga, or Ge and As are known in addition to Zn.sup.30.
In the conventional methods as described above, it is difficult to form a micropatterned p-n region on the order of 2 .mu.m or less. For example, in the diffusion method, since leakage of the diffusion material occurs under the mask, the size of the mask cannot be reduced much.
In the ion-implantation method, even if the implantation region can be narrowed, impurity diffusion occurs by annealing after implantation, and an impurity region is expanded beyond the implantation region. Therefore, it is also difficult to form a micropatterned region. Furthermore, in the ion-implantation method, if a GaAs substrate is employed, defects are formed due to evaporation of As and Ga. Therefore, a high-purity film cannot be formed.
A detailed structure of a conventional semiconductor laser will now be described.
FIG. 3 is a view showing a schematic structure of a conventional semiconductor laser, and FIG. 4 is a graph showing a refractive index distribution of the laser shown in FIG. 3.
Referring to FIG. 3, a 0.5-.mu.m thick n-GaAs buffer layer 2 is formed on an n-GaAs substrate 1, and a 1.5-.mu.m thick n-Ae.sub.x Ga.sub.y As (x=0.3, y=0.7) clad layer 3 is formed on the buffer layer 2.
A 0.1-.mu.m thick nondoped GaAs active layer 4, a 1.5-.mu.m thick p-Ae.sub.x Ga.sub.y As (x=0.3, y=0.7) clad layer 5, and a 0.5-.mu.m thick p-GaAs contact layer 6, an Au-Ge electrode 7, and an Au-Sn electrode 8 are sequentially formed.
When a voltage is applied to the semiconductor laser having the above-mentioned structure, a current supplied from the electrode 7 is diverged as indicated by arrows 9 before it reaches the electrode 8. For example, if the electrode 7 has a thickness of about 5 .mu.m, the current is diverged to 4 to 5 times, as indicated by the arrows 9, and a current required for oscillation is also increased (about 150 to 200 mA).
In this related art, as shown in FIG. 4, a refractive index n of the active layer is about 3.65, and the refractive index n of the clad layers is about 3.4. Therefore, optical confinement in the vertical direction can be attained by the clad layers 3 and 5. However, optical confinement in the vertical direction cannot be performed. For this reason, light is scattered in a wider range, and hence, a differential quantum effect is degraded.
In order to eliminate the above problem, the following laser structure has been proposed.
FIGS. 5 and 6 are schematic views showing structures of other conventional semiconductor lasers.
In FIG. 5, an n-type buffer layer 12, an n-type clad layer 13, a nondoped active layer 14, and a p-type clad layer 15 are sequentially formed on an n-type substrate 11. Furthermore, an n-GaAs current block layer 16 having an opening, a p-type clad layer 17, a p-type contact layer 18, and electrodes 19 and 20 are formed on the structure.
When a voltage is applied to the structure described above, a current flows from the electrode 19 to the electrode 20. In this case, since it is not easy for a current to flow through a portion of the current block layer 16, the current is concentrated on the opening of the current block layer 16, as indicated by arrows 22.
If a distance 21 between the current block layer 16 and the active layer 14 is set to be a distance causing light absorption in the current block layer 16 in consideration of light leakage from the active layer 14, a light emitting portion is limited to the opening of the current block layer 16. Thus, a laser oscillation can be performed at a low threshold value (typically, 30 to 40 mA).
In the detailed ratings of the laser structure, the distance 21 is 0.4 .mu.m, the thickness and the carrier concentration of the current block layer 16 are respectively 0.6 .mu.m and 6.times.10.sup.18 cm.sup.-3, and the carrier concentration of the p-type clad layers 15 and 17 is 1 .times.10.sup.18 cm.sup.-3. In addition, the opening of the current block layer 16 has a width of 3 .mu.m.
In the semiconductor layer shown in FIG. 6, an n-type buffer layer 32, a clad layer 33, a nondoped active layer 34, p-type clad layers 35 and 36, an n-GaAs current clock layer 41 having an opening, a contact layer 37, and electrodes 40 and 38 are formed on an n-type substrate 31 having a stepped portion.
In this structure, a current is also concentrated on the opening of the current block layer 41 (arrows 39), and a refractive index difference is present in the horizontal direction due to the stepped portion. Therefore, light is confined in the stepped portion, and high efficiency can be achieved.
However, in the semiconductor laser, in order to manufacture the current block layer (16, 41), after the n-GaAs layer is formed on the entire surface, the opening must be formed by chemical etching.
For this reason, defects may be formed by etching or by attachment of dust when the structure is taken outside a deposition chamber.
Furthermore, since the current block layer (16, 41) is formed adjacent to the active layer (14, 34), the etching process cannot be precisely controlled. In particular, in the laser shown in FIG. 6, since the structure has an inclined portion, it is difficult to control the etching process.
Since the semiconductor layer must be grown before and after the etching, the manufacturing processes become complicated, and a manufacturing cost is increased.
FIG. 7 is a schematic view showing a structure of a conventional BMQW laser (Buried multiquantum well lasers). The BMQW laser is reported in, e.g., Appl. Phys. Lett. 45(1), 1 July 1984.
The structure shown in FIG. 7 includes a p-GaAs substrate 82, a p-Ae.sub.xl Ga.sub.yl As (xl=0.35 and yl=0.65, and these values apply to the following parameters) buffer layer 83, a p-Ae.sub.x Ga.sub.y As clad layer 84, a region 85 which is disordered by Zn diffusion, a Zn diffusion region 86, an SiO.sub.2 current block layer 87, an AuGeNi-Au electrode 88, a Cr-Au electrode 81, an n-Ae.sub.xl Ga.sub.yl As upper clad layer 89 in which no Zn is diffused, an n-GaAs contact layer 90, and a BMQW active layer 91.
The feature of this laser is as follows. When the region 85 as the BMQW active layer is disordered upon Zn diffusion, the texture of the disordered region 85 is changed to cause a change in refractive index, so that light is confined in the active layer 91. Since a p-n junction is formed in the horizontal direction upon Zn diffusion, a current can be concentrated. In addition, the laser is oscillated at a low threshold level current of 33 mA.
However, in this structure, processes after Zn diffusion are required, and technical problems such as poor controllability remain unsolved.
FIG. 8 is a schematic view of a conventional BH laser (Buried-heterostructure injection lasers). The BH laser is reported in, e.g., Journal of Applied Physics, Vol. 45, No. 11, November 1974, 4899.
The structure shown in FIG. 8 includes an n-GaAs substrate 102, an n-AeGaAs layer 103 for confining a current, an oxide film 104, electrodes 101 and 105, a p-AeGaAs clad layer 106, a GaAs active layer 107, and an n-AeGaAs lower clad layer 108.
In the method of manufacturing the laser with the above structure, the lower clad layer 108, the active layer 107, and the clad laser 106 are formed on the substrate 102. Thereafter, the resultant structure is etched to remove a portion other than the central portion, thereby re-growing the n-AeGaAs layer 103 for confining the current.
This laser exhibits a very low threshold level current of 15 mA. However, the manufacturing processes are complex, as described above.
Appl. Phys. Lett. Vol. 47, No. 12, 15 December 1985 discloses a method of forming a lateral p-n junction without using the above diffusion method and the like. This method utilizes the fact that a semiconductor laser including a convertible impurity has a different conductivity type depending on the orientation of a deposition surface.